Webb optical alignment complete and virtually perfect

An infrared image of a star field from the now-retired Spitzer Space Telescope is shown at left compared with a view of the same field in the Large Magellanic Cloud taken with the James Webb Space Telescope’s Mid-Infrared Instrument, or MIRI. Image: NASA/JPL-Caltech (left), NASA/ESA/CSA/STScI (right)

Engineers have completed alignment of the James Webb Space Telescope’s optical system, achieving better than expected performance, to kick off two months of instrument checkout and calibration across 17 modes of operation. The first public-release imagery, intended to showcase the observatory’s performance, is expected in mid July.

While one seldom hears a scientist or engineer describe a physical system as “perfect,” that’s exactly the term used by Michael McElwain, Webb project scientist at NASA’s Goddard Space Flight Center, during a 9 May teleconference.

“I’m delighted to report that the telescope alignment has been completed with performance even better than we had anticipated,” McElwain told reporters. “We basically reached a perfect telescope alignment. There’s no adjustment to the telescope optics that would make material improvements to our science performance.”

While there is always a risk of problems down the road, “I have great confidence that we’ll get to the finish line, and we’ll have a terrific science mission with tremendous scientific discovery in the next few months. So I’m just super excited to be at this point.”

On 18 April, test images were released showing razor-sharp stars as seen by JWST’s four instrument. On 9 May, a new image was posted showing two views of a star field in the Large Magellanic Cloud, one from the now-retired Spitzer Space Telescope and the other from Webb’s Mid-Infrared Instrument, or MIRI.

Stars in the Spitzer image, at the limit of the telescope’s resolution, were blurred with hints of nebulosity. Webb’s view shows pinpoint stars across the field and clearly defined clouds and filaments.

Test images from Webb’s four primary instruments, released 18 April, show razor-sharp focus across the fields of view. Image: NASA/ESA/CSA/STScI

“From a sort of an intellectual standpoint, you can appreciate that the images from Webb are going to be better because we have 18 (mirror) segments, every one of which is larger than the single segment that formed the Spitzer telescope’s mirror,” said Marcia Rieke, principal investigator for Webb’s Near-Infrared Camera, or NIRCAM.

“It’s not until you actually see the kind of image that it delivers that you really internalise and go, wow! Just think of what we’re going to learn! Spitzer taught us a lot, but this is like a whole new world. Just unbelievably beautiful.”

The team now plans to spend the next two months Webb’s four science instruments through their paces, collecting test images and spectra to verify 17 different operating modes of operation before beginning science observations later this summer.

But first, NASA plans to unveil a series of “early release observations,” or EROs, to showcase Webb’s scientific prowess and in so doing, help justify the observatory’s $10 billion price tag.

The list of potential targets is a closely held secret, but NASA plans to unveil the selected ERO images and spectra in mid July.

“Their objective is to demonstrate … to the world and to the public that Webb is fully operational and that it produces excellent results,” said Klaus Pontoppidan, Webb project scientist at the Space Telescope Science Institute in Baltimore. “It’s also an opportunity to celebrate the beginning of many years of Webb science.”

He said the targets, selected by a committee of experts, will showcase all four science instruments “to highlight all the Webb sciences themes … from the early universe, to galaxies over time, to the lifecycle of stars, and to other worlds.”

“Overall, the observatory performance has been phenomenal,” McElwain said. “We’re really in the homestretch.”